1. Field of the Invention
The present invention relates to a process for separating water from an aqueous solution of an organic substance or a vapor of a mixture of an organic substance and water. More particularly, the present invention relates to a membrane used for separating and concentrating an aqueous solution of an organic substance by pervaporation or a vapor of a mixture of an organic substance and water by vapor permeation and a process for producing said membrane.
2. Prior Art
In the field which relates to the concentration of an aqueous solution of an organic substance having a low concentration or separation thereof by means of a membrane, a reverse osmosis method has been employed in practice. However, since the reverse osmosis method requires the application of a pressure which is higher than the osmotic pressure of the solution to be treated, this method cannot be employed in the treatment of an aqueous solution having a high concentration and a high osmotic pressure. Thus, the concentration of the solution which can be treated is limited.
The pervaporation method and vapor permeation method have attracted attention as new methods of separating water without being influenced by the osmotic pressure. In the pervaporation method, the solution to be treated is fed onto the primary side of the membrane and the pressure on the secondary side (permeation side) of the membrane is reduced or, alternatively, a gaseous carrier is blown therein to permeate the substance to be separated in a gaseous phase through the membrane. The vapor permeation method is different from the pervaporation method in that the vapor mixture is fed onto the primary side of the membrane. The substance permeated through the membrane is collected by cooling and thereby condensing the permeated vapor. There are various reports on the pervaporation method. For example, for the treatment of an aqueous ethanol solution, a homogeneous cellulose acetate membrane is disclosed in the specification of U.S. Pat. No. 2,953.502 and a polyvinyl alcohol membrane is reported in U.S. Pat. No. 3,035,060. The separation factors in both of these processes are low. Though a composite membrane comprising a skin layer which comprises a cellulose acetate film or a polyvinyl alcohol film is disclosed in the specification of Japanese Patent Laid-Open No. 109204/1984 and a crosslinked composite polyethyleneimine membrane is disclosed in the specification of Japanese Patent Laid-Open No. 55305/1984, only a low permeation velocity or separation factor is obtained by their use. A membrane made of an anionic polysaccharide is disclosed in the specification of Japanese Patent Laid-Open No. 129104/1985. Since the raw materials for the membranes shown in the examples in said specification are water-soluble polymers, the durability of the membrane is poor in the treatment of an aqueous solution of an organic substance having a low concentration. It is described therein that the crosslinking is effected sufficiently for insolubilizing the membrane in water, although this process is not shown in the examples. However, usually, the permeation velocity is reduced, even though the separation factor is increased by the crosslinking treatment, as will be shown in the comparative examples below.
As described above, a large surface area is required of the separation membrane used in the conventional pervaporation and vapor permeation methods because of a low permeation velocity. Further, it is necessary to circulate the permeated liquid of a high concentration so as to concentrate the liquid to an intended concentration because of a low separation factor. Therefore, the costs of the apparatuses and operations are disadvantageously high in the conventional processes.